1. Field of the Invention
The present invention relates to an engine controller including an air/fuel (A/F) ratio adjusting unit, such as a throttle valve and a fuel injector valve, for adjusting an A/F ratio of an air-fuel mixture subjected to combustion, and an A/F ratio detecting unit, such as a linear A/F ratio sensor, disposed in an exhaust passage. More particularly, the present invention relates to an engine controller capable of diagnosing, for example, whether the A/F ratio detecting unit has deteriorated or not, and optimizing A/F ratio control in accordance with the diagnosis result.
2. Description of the Related Art
Recently, controls on auto-emission have been tightened. To clean HC, CO and NOx exhausted from an engine, it has become general to dispose, in an exhaust passage, a three-way catalyst and, upstream of the catalyst, a linear A/F ratio sensor (hereinafter referred to as an “A/F sensor”) producing a linear output (signal) with respect to an A/F ratio so that the catalyst develops an action with high efficiency and A/F ratio feedback control is performed with high robustness. Meanwhile, self-diagnosis controls have also been introduced in North America, Europe, Japan, etc. Correspondingly, there arises a demand for increasing diagnosis accuracy of the A/F sensor, i.e., for identifying a deterioration mode (gain deterioration or response deterioration) of the A/F sensor and detecting a degree of the deterioration with high accuracy. Under such a background, proposals have hitherto been made on a method (diagnosis method) for detecting the deterioration of the A/F sensor with high accuracy, and a method for optimizing parameters in the A/F ratio feedback control in accordance with the diagnosis result, to thereby maintain the performance of an exhaust cleaning system.